In the case of patients with neurological or psychiatric diseases, e.g. Parkinson's disease, essential tremor, dystonia, obsessive disorders, nerve cell networks exhibit pathological (abnormal) activity, e.g. excessively synchronous activity, in circumscribed regions of the brain, e.g. in the thalamus and the basal ganglia. In this case, a large number of neurons synchronously form action potentials, i.e. the involved neurons fire excessively synchronously. By contrast, in healthy individuals the neurons fire in a qualitatively different fashion in these regions of the brain, e.g. in an uncorrelated fashion.
In the case of Parkinson's disease, the pathologically (abnormally) synchronous activity changes the neuronal activity in other regions of the brain, e.g. in areas of the cerebral cortex such as the primary motor cortex. Here, the pathologically synchronous activity in the region of the thalamus and the basal ganglia impresses its rhythm onto the areas of e.g. the cerebral cortex, and so, ultimately, the muscles controlled by these areas develop a pathological activity, for example rhythmic trembling (tremor).
Neurological and psychiatric diseases with excessively pronounced neuronal synchronization are currently treated by electrical brain stimulation if medicinal therapy fails.